


One Night Shenanigans

by Dark_Crystal_Demon



Category: Undertale (Video Game)
Genre: Fontcest, Multi, Undertale Shipping Challenge -Sept 30
Language: English
Status: Completed
Published: 2016-09-30
Updated: 2016-09-30
Packaged: 2018-08-18 16:25:52
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 2,083
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8168431
Author URL: https://archiveofourown.org/users/Dark_Crystal_Demon/pseuds/Dark_Crystal_Demon
Summary: Saw this post by the amazing spoopy-gaster on Tumblr: "know what i’m thirsty forfontcest shenanigans at a sleepover with alphys and undyne."I can't get the images out of my head: Alphys/Undyne... Sans/Papyrus... Sleepover!Get ready for a wild ride as Undyne delivers to Alphys one of the greatest birthday presents of all: Fontcest... and hot and heavy Undyne/Alphy time! Warning: graphic sexual descriptions





	

Sweat drips from Undyne's body as she trains on the heavy bags. Each punch is precise, deadly, and a complete turn on. Alphys watches from the door. She's wondering how she got so lucky. Then Undyne cocks her head back to look at her lover with the same appreciative gaze.

"Good morning, Birthday Girl!" Undyne yells, flexing her muscles with mock seriousness, "What wishes can I grant you today?"

Alphys giggles and blushes more out of habit than actual embarrassment at the innuendo. Years ago, she would be a stuttering mess at the suggestion of anyone being attracted to her, but now...

Alphys starts to unbutton her night gown. Revealing her beautiful curves and full reptilian breasts, she looks up to see Undyne already walking towards her. The rough fabric of her fighting gloves brushes across Alphys' nipples and she moans into her girlfriend's mouth.

"T-that feels amazing." 

Undyne continues to fondle her. She enjoys when Alphys' breath comes out in short hisses when she gets aroused. Her fingers wrap around the elastic band of her sports bra, but Undyne won't let her take it off. 

"Today is all about you, beautiful," Undyne picks her up effortlessly and lays her on the workout bench. She stands between her lovers legs and admires her half-naked body sprawled out before her. She bends down to continue caressing Alphys' breasts and bury her head between her thick thighs. 

Undyne licks her wet underwear above the area of her clit. 

"Sssss," Alphys hisses through her teeth. Undyne continues to lick and enjoys Alphys' thighs squeezing around her head. "Ssssoooo g-good!"

Undyne fondles her breasts in rhythm to her tongue and rides her girlfriend's body as Alphys undulates in waves of pleasure. Undyne tears at the restrictive panty fabric with her teeth and Alphys starts to moan louder as Undyne's tongue touches her sensitive, slick scales. Undyne drinks her in and explores her deep recesses.

Undyne brings one hand down and takes off the glove with her mouth. She kisses Alphys' clit between every movement and tweaks her nipples. As Alphys jerks her hips, Undyne follows her down with her hand and glides two fingers inside her. She was so tight and squeezes around her. 

Undyne brings her mouth back down over Alphys' clitorus and curls her fingers forward to tap her inner spot. Alphys grips the edges of the bench and lurches her pelvis. Undyne is ready for this and rides the wave; her tongue and fingers never stopping. 

She speeds up and then slows down. She wants to keep savoring the taste of her lover. She adds another finger and enlists her other hand to grab Alphys' plump, soft ass. 

Undyne's wet too, but she takes that energy and sends it to her tongue. Alphys is getting close and Undyne starts to suck ever so gently on her clit along with the curling of her fingers. 

"AH!"

She sucks a little more, swirling her tongue. 

"AH!"

Her thighs are almost suffocating Undyne now. She sucks more and Alphys shakes uncontrollably. 

"AAAAAAAAHHHHHHHH!!!!"

Undyne stops her mouth as Alphys comes and lightly licks her clit to keep the pleasure rolling. Alphys hugs her with her inner muscles and thigh. Undyne kisses her clit and then pulls out her fingers and licks them. "Happy Birthday."

"Best gift ever."

"Oh, that was just my 'Good Morning' card. The gift has yet to arrive," Undyne smirks as Alphys sits up and fingers her ripped underwear. She had really liked this pair, but it was inevitable. Undyne loved ripping clothing.

"What's my present?" Alphys asks, casually buttoning up her gown. Undyne stops her by slipping her hands back into her clothes. She couldn't get enough of those big tits. Alphys bites her lip in another small hiss. 

"I'm not telling," Undyne whispers in her ear as she continues to squeeze Alphys' breasts in her hands.

Undyne kisses her just behind the ear and feels Alphys' hips jerk once again against her own. "The Birthday Girl has more wishes already?"

"Your the one rubbing the bottle," Alphys says playfully. Undyne bites Alphys' neck gently. Alphys pretends to protest, "We have to get ready for guests."

"You're right," Undyne scoops up Alphys in her arms. The gills on her neck flair for a second. "Let's take this to the shower."

***

When Alphys walks into the kitchen, she's surprised by how many bottles of alcohol are on the counter. 

"'Dyne," Alphys yells into the house. "How many people are we expecting?"

Undyne walks into the room, pulling her shirt over her chiseled abs. There's a mischievous glint in her eye as she looks at the bottles.

"It's part of your birthday present." Alphys looks at her quizzically. "No, I'm not telling you, but let's just say I learned something interesting at Grillby's about a certain pair of skelebrothers."

Alphys is not expecting that answer and starts asking for more details as the door bell rings. Guests start coming in, and the conversation is forgotten for a while. 

The brothers previously mentioned arrive fashionably late but bring burgers and more beer. Undyne plays hostess with very generous amounts of booze to see if the rumors were true. Only time will tell.

Undyne starts a drinking game or five. Papyrus and Sans start to get closer and closer as the night continues. Undyne smiles at their drunken grins. 

Other guests filter out. Mettaton is, of course, one of the hardest guests to convince to leave. Undyne knows he's Alphys favorite creation, but she just can't stand all the posing with the legs. She eventually has a leg standoff with the robot and wins the right to send him home. 

Finally she closes the door on the last of the guests aside from the skeletons. 

"Hey Sans," Undyne says, sitting bon the opposite couch with Alphys. "Since it's Alphys' birthday, I was wondering if you and Papyrus could make one of her dreams come true."

"ANYTHING FOR ALPHYS AND MY CAPTAIN!" Papyrus yells, fully inebriated and grinning from temporal bone to temporal bone. Sans looks suspicious, but Undyne is hoping that the gossip she heard from the dogs is true...

"You guys should kiss!"

"WHAT?!? WHY?!?" Papyrus is shocked while Alphys spit her drink onto the table and stutters, "U-un-D-dyyyyyyNee!"

"Come on. We're all friends! No judgement here! Alphys loves her yaoi and you two would make a great pairing."

Sans smirks; the lights in his eyes flashing and the desire to keep secrets disappearing, "You're right." He pulls his brother down by his red scarf and looks him in the eye. "We do make a great couple."

He kisses Papyrus and holds him down to his lips with his scarf. Magical tongues form in their mouths and start dancing and twisting around each other. It's hard for Papyrus to think with all the booze and the skill of Sans' tongue. He knew what that tongue could do... but... "NO! SANS! NOT AGAIN! WE PROMISED IT WAS THE LAST TIME!"

"I crossed my bones behind my back," Sans counters, caressing Papyrus' atlas vertebra near the base of his skull. Papyrus purrs under the touch and leans his skull into Sans' hand. "Who knows a skeleton better than another skeleton? I want you."

"Sans..." Papyrus' voice drops in desire.

"Papyrus..." Sans pulls his brother back into a kiss that doesn't end. It becomes more urgent and they start taking off each other's clothes: kissing, caressing, and nibbling on bones with a skilled familiarity of the each other's sensual weaknesses.  

"Is this really happening?" Alphys whispers, head swimming with drink as she crawls into Undyne's lap. Undyne can't believe it herself, but then she feels Alphys start to touch herself. Her own inhibitions lost in the heat of the scene before her and the alcohol in her veins. 

Undyne adjusts Alphys in her lap. Alphys wiggles her tail underneath herself and between Undyne's legs. She presses it against Undyne's sex, and Undyne moans. They pleasure themselves while watching Sans and Papyrus grind against each other, magical cocks growing between them.

The women's actions weren't going unnoticed, but they found the sounds of their moans even more arousing.

"We've never had an audience before," Papyrus comments as he rubs the sensitive section of Sans' sixth rib.

Sans looks over at the other couch. "They're naughty girls, Paps. They like watching guys touch each other," Sans chuckles. "Let's put on a show they won't forget."

Sans guides Papyrus to stand with his spinal cord to the other couch where Alphys and Undyne sat. Alphys catches her breath as Sans keep eye contact around his brother's vertebrae. His eyes don't leave hers as he takes his brother's cock in his mouth.

It reminds Alphys of old nights in the lab... before her and Undyne had ever met. The experimentations... the lab table... her hands tied above her head as he looked at her between her legs...

"SANS!" Papyrus moans and grabs his brother's skull. Sans closes his eye sockets and concentrates on swirling his magical tongue around his dick. He grows his tongue longer and thicker. He trails it down his brother's shaft and into his pelvic bone. Papyrus' tibia are getting weak, and he holds onto Sans' shoulders for support. He starts thrusting into his brother's skull and magical cum mixes with saliva to create a slippery blue scene. 

Across the room on the other couch, Undyne has unzipped her pants and Alphys becomes distracted by her wetness. She slips her tail into Undyne and starts to follow of rhythm of Sans' mouth on Papyrus' cock. Undyne stretches with the increasing thickness of Alphys' tail as she plunges it deeper and deeper into her. 

"I'm glad you're enjoying this, lover," Undyne gasps and takes the rests of Alphys' tail inside her. Alphys shudders as Undyne takes over for her hand and let's her enjoy the show while nibbling on her neck. "I remembered how much you liked skeletons."

Alphys and Papyrus moan as their partners pleasure them with increased speed. Alphys world narrows to Undyne's hand and her walls surrounding her tail. Papyrus can't get enough of Sans' electric blue tongue squeezing him to the point of estacsy. 

"NYEEH" Papyrus comes into his brother's mouth. Cum trailing down the inside of Sans' spine and joining with his own erection. 

Sans licks away the stream and Papyrus bends down to take care of his brother's cock, but Sans spins him around; bending him over the coffee table. Papyrus moans as Sans dick grows to fill his pelvic floor. 

The view of Sans thrusting into Papyrus sets Alphys over the edge. She orgasms into Undyne's hand which causes her tail to stiffen inside Undyne and trigger her own orgasm.

Sans enjoys the view of Alphys' tits heaving in her unbuttoned shirt while he thrusts against Papyrus' bones. The lovers cuddle and continue to fondle each other with the show. 

He grows bigger at the thought and pushes against the walls of Papyrus' hips. His brother whimpers and moans with the pressure. It drives Sans to the brink, but he doesn't want to come this way. He says, "I want to see your face."

Their maneuvering takes them to the ground where Sans grinds his patella into the carpet as he thrusts deeply into Papyrus. 

"Sans!" Papyrus repeats his name over and over as Sans slams his cock into his coccyx and drives him to a skeletal orgasm. 

"Sans!" He's losing his mind and Sans thrusts harder.

"Sans!" He can feel it coming...

"Sans!" He's close!

"SANS!" 

"PAAAAAA!" He comes all over his brother's bones that are shaking from their own orgasm. 

The magic dissipates and Sans collapses in the arms of his brother. The faint noise of slurping and moans float across the room as the women eat each other out on the couch.

Sans chuckles. What a weird night.

He snuggles against Papyrus and realizes they just officially came out to their friends. 

"Was this the last time?" Papyrus asks softly. 

"Never." Sans answers, then jokes, "Tomorrow let's fuck in front of Grillby."

Papyrus laughs and pulls his brother closer. "Let's come out to the River Person too. I've always wanted to fuck on a boat."

Unaware of the sweet scene just a few feet away, Undyne and Alphys match each other's rhythms and cum into their eager mouths. 

Alphys looks at the couple lying on the ground and smiles. Undyne wraps her arms around her and kisses her cheek. "Happy Birthday, Alphy."

 

**Author's Note:**

> Thanks again to [spoopy-gaster](http://spoopy-gaster.tumblr.com/) for the inspiration!
> 
> Check Out My Tumblr (NSFW): [Dark_Crystal_Demon](https://www.tumblr.com/blog/darkcrystaldemon)
> 
> And if you really like my style, subscribe to me at [Dark_Crystal_Demon](http://archiveofourown.org/users/Dark_Crystal_Demon), to get updated when I make new fics or chapters!


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